The present invention is directed to an integrated process for manufacturing Fischer-Tropsch products and acetic acid, methyl acetate, or acetic anhydride from synthesis gas preferably derived from reforming methane or natural gas.
Processed natural gas, consisting essentially of methane (typically 85-95 volume percent) but with significant amounts of carbon dioxide present in some areas, may be directly used as a clean burning gaseous fuel for industrial heat and power plants, for the production of electricity, and to fire the kilns in the cement and steel industries. It is also useful as a chemical feedstock, but large-scale use for this purpose is largely limited to conversion to synthesis gas which in turn is used for the manufacture of methanol and ammonia. It is notable that for the foregoing uses, no significant refining is required except for those instances in which the wellhead-produced gas is sour, i.e., it contains excessive amounts of hydrogen sulfide. Natural gas, however, has essentially no value as a portable fuel at the present time. In liquid form, it has a density of 0.415 and a boiling point of minus 162xc2x0 C. Thus, it is not readily adaptable to transport as a liquid except for marine transport in very large tanks with a low surface to volume ratio, in which unique instance the cargo itself acts as refrigerant, and the volatilized methane serves as fuel to power the transport vessel. Large-scale use of natural gas often requires a sophisticated and extensive pipeline system.
A significant portion of the known natural gas reserves is associated with fields found in remote regions where it may not be economical to transport the gas to market. For many of these remote fields, pipelining to bring the gas to potential users is not economically feasible. In such circumstances, it is desirable to convert the methane in the natural gas at the production site into more valuable and more easily transported products. Currently, one of the most important products made from methane is acetic acid. Products prepared from methane using the Fischer-Tropsch process also are becoming increasingly more important. Both acetic acid and Fischer-Tropsch products are prepared by first reforming methane into synthesis gas or syngas which is primarily a mixture of carbon monoxide and hydrogen. Synthesis gas may contain varying amounts of carbon dioxide, water, and unconverted light hydrocarbon feedstock. Impurities originally present in the natural gas, such as sulfur and nitrogen, may also be present. The methane may be reformed using oxygen according to the general formula:
2CH4+O2xe2x86x922CO+4H2xe2x80x83xe2x80x83(1)
In addition, the methane may be reformed to synthesis gas using steam
CH4+H2Oxe2x86x92CO+3H2xe2x80x83xe2x80x83(2)
or by using carbon dioxide.
CH4+CO2xe2x86x922CO+2H2xe2x80x83xe2x80x83(3)
While methane is the most important source of synthesis gas, other sources of synthesis gas have been described, as, for example, by the gasification of coal or the decomposition of methanol.
In the manufacture of acetic acid, the synthesis gas is first converted to methanol by an equilibrium reaction over a catalyst usually containing copper. This process step may be shown as:
CO+2H2⇄CH3OHxe2x80x83xe2x80x83(4)
Typically there will be about a 20% to 50% synthesis gas conversion per pass to methanol in this step. Carbon dioxide which is often present in significant amounts in the synthesis gas will also react with hydrogen to form methanol as shown in the following:
CO2+3H2xe2x86x92CH3OH+H2Oxe2x80x83xe2x80x83(5)
The methanol in turn is reacted with unreacted carbon monoxide in the presence of a suitable catalyst to make acetic acid in what is referred to in this disclosure as a carbonylation reaction. As noted above, reaction (4) is an equilibrium reaction, and the reverse reaction, i.e. the decomposition of methanol, may be used to generate a source of synthesis gas. The carbonylation reaction may be represented as follows:
CO+CH3OHxe2x86x92CH3COOHxe2x80x83xe2x80x83(6)
Unreacted hydrogen which passes along with the methanol and CO in the feed to the carbonylation reaction has generally been described as undesirable in the literature in amounts in excess of about 2% by weight. This is due to the alternative and generally favored reaction between carbon monoxide and hydrogen to reform methane which effectively lowers the yield of acetic acid. See for example European Patent Specification EP 0 526 974 B1 and U.S. Pat. No. 5,659,077. Accordingly, commercial processes using this route incorporate an intermediate separation step to remove the excess hydrogen from the feed prior to passing it into the carbonylation reactor. However, U.S. Pat. Nos. 5,189,203 and 5,286,900 suggest that this energy intensive gas separation step may be avoided by use of a rhodium catalyst and an methyl iodide promoter. In this instance, the hydrogen passes through the carbonylation step.
Products prepared from the Fischer-Tropsch process comprise a mixture of various liquid and solid hydrocarbons which are generally up-graded to higher value products such as lubricating base oils and transportation fuels. Upgrading processes for Fischer-Tropsch products include, but are not necessarily limited to, hydrogenation, hydrocracking, reforming, catalytic isomerization, hydrofinishing, and hydrotreating processes, all of which require hydrogen. One source of hydrogen which is commonly used in association with the Fischer-Tropsch process is by means of the water gas shift reaction (7) in which carbon monoxide recovered from the synthesis gas along with added water react to produce hydrogen gas and carbon dioxide.
CO+H2Oxe2x86x92CO2+H2xe2x80x83xe2x80x83(7)
In addition the tail gas from a Fischer-Tropsch plant usually contains large amounts of carbon dioxide as a result of the water gas shift reaction-taking place in the Fischer-Tropsch reactor. The carbon dioxide resulting from the water gas shift reaction to generate hydrogen and the tail gas from the Fischer-Tropsch reactor is usually recycled to the syngas reformer or, alternately, vented to the atmosphere as waste gas. Since carbon dioxide is one of the most significant green house gases, it is advantageous to find a way to either avoid generating carbon dioxide or use it to produce valuable products.
As used in this disclosure the words xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d is intended as an open-ended transition meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrase xe2x80x9cconsists essentially ofxe2x80x9d or xe2x80x9cconsisting essentially ofxe2x80x9d is intended to mean the exclusion of other elements of any essential significance to the composition. The phrases xe2x80x9cconsisting ofxe2x80x9d or xe2x80x9cconsists ofxe2x80x9d are intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.
The present invention is directed to an integrated process for manufacturing methanol, acetic acid, and Fischer-Tropsch products from synthesis gas without the need for a separate hydrogen generation step while reducing the production of carbon oxides as waste gas. Accordingly, the present invention may be described as an integrated process for making upgraded Fischer-Tropsch products and a carbonylation product which comprises one or more of acetic acid, methyl acetate, and acetic anhydride from synthesis gas comprising the steps of (a) separating the synthesis gas into a first portion and a second portion; (b) reacting the hydrogen and carbon monoxide from the first synthetic gas portion in a reaction zone in the presence of a catalyst under conditions preselected to form methanol; (c) recovering an intermediate product mixture comprising methanol, hydrogen and carbon monoxide from the reaction zone of step (b), wherein said intermediate reaction mixture contains at least 25% by volume of hydrogen; (d) contacting the intermediate product mixture recovered in step (c) in a carbonylation reaction zone under vapor phase conditions with a carbonylation catalyst containing one or more of a metal selected from the group consisting of rhodium, iridium, osmium, and cobalt on an inert support and a halide promoter under carbonylation conditions selected to produce a carbonylation product comprising one or more of acetic acid, methyl acetate, and acetic anhydride; (e) recovering separately hydrogen and the carbonylation product from the carbonylation reaction zone of step (d); (f) contacting the second synthetic gas portion of step (a) with a Fischer-Tropsch catalyst in a Fischer-Tropsch reaction zone under conditions preselected to produce Fischer-Tropsch products; (g) feeding the hydrogen recovered from step (e) into a hydroprocessing zone for upgrading Fischer-Tropsch products; (h) upgrading the Fischer-Tropsch products of step (f) in the hydroprocessing zone of step (g); and (i) recovering an upgraded Fischer-Tropsch product. The preferred source of the synthesis gas is by reforming methane or natural gas by means of one or more of reactions (1) or (2) or (3), above. However, one skilled in the art will recognize that other sources of synthesis gas may also be used to carry out the invention.
In another embodiment of the present invention, tail gas from a Fischer-Tropsch plant is employed to produce the methanol used to manufacture acetic acid in the carbonylation step. Tail gas from a Fischer-Tropsch plant typically will contain unreacted carbon monoxide and hydrogen as well as significant amounts of carbon dioxide which are formed by the water gas shift reaction which takes place as a side reaction in the Fischer-Tropsch reactor. Accordingly, the present invention is also directed to an integrated process for making upgraded Fischer-Tropsch products and a carbonylation product which comprises one or more of acetic acid, methyl acetate, and acetic anhydride from the tail gas from a Fischer-Tropsch plant which comprises a gaseous mixture of carbon oxides and hydrogen, said process comprising the steps of (a) recovering the tail gas from the Fischer-Tropsch plant and using it as feed to a methanol plant; (b) reacting the hydrogen and carbon oxides from the tail gas in a reaction zone of the methanol plant in the presence of a catalyst under conditions preselected to form methanol; (c) recovering an intermediate product mixture comprising methanol, hydrogen and carbon oxides from the reaction zone of step (b), wherein said intermediate reaction mixture contains at least 25% by volume of hydrogen; (d) contacting the intermediate product mixture recovered in step (c) in a carbonylation reaction zone under vapor phase conditions with a carbonylation catalyst containing one or more of a metal selected from the group consisting of rhodium, iridium, osmium, and cobalt on an inert support and a halide promoter under carbonylation conditions selected to produce a carbonylation product comprising one or more of acetic acid, methyl acetate, and acetic anhydride; (e) recovering separately hydrogen and the carbonylation product from the carbonylation reaction zone of step (d); (f) feeding the hydrogen recovered from step (e) into a hydroprocessing zone for upgrading Fischer-Tropsch products; (g) upgrading Fischer-Tropsch products from the Fischer-Tropsch plant in the hydroprocessing zone of step (f); and (h) recovering an upgraded Fischer-Tropsch product. As used in this disclosure the term xe2x80x9ccarbon oxidesxe2x80x9d refers to carbon dioxide and carbon monoxide. One skilled in the art will recognize that both carbon dioxide and carbon monoxide may be used in the production of methanol according to step (b) and as shown in reactions (4) and (5), however, only unconverted carbon monoxide participates in the carbonylation step, i.e. reaction (6) above.
The preferred carbonylation catalyst will contain rhodium or iridium as the active metal with rhodium being most preferred. The preferred halide promoter is methyl iodide or methyl bromide, with methyl iodide being especially preferred.
The carbonylation product recovered from the carbonylation is selected from any one of the group consisting essentially of acetic acid, methyl acetate, and acetic anhydride. In most cases the carbonylation product will comprise a mixture of two or more of these products, most especially a mixture of acetic acid and methyl acetate. Both methyl acetate and acetic anhydride are readily converted to acetic acid by processes recorded in the literature and familiar to those skilled in the art.